User blog comment:Bossthebeast123/Tips for catching mythicals/Legends/@comment-27248965-20170116013601

I'd purchase a lot of Dusk Balls and Quick Balls. Dusk Balls have 3.5x catch rate overnight or in caves, and are great assets when catching Jirachi, as well as any other future Legendary / Mythical Pokemon appearing overnight or in caves. As for Quick Balls, they are helpful when capturing some easier-to-catch Legendary / Mythical Pokemon like Shaymin (catch rate 45), as well as some Roaming Pokemon in the future. Net Balls may also be useful when catching Palkia and Genesect later on, because these kind of Poke Balls triple the catch rate on Water and Bug Pokemon.

Unless you are very rich and there isn't any other options available, I don't recommend buying the Elemental Balls, because they cost double the price of Quick and Dusk Balls, while only having the same catch rate as a maximum effect Dusk Ball.

Instead of Focus Sash or Weakness Policy, I'd rather send out a Pokemon immune to or resist the Legendary / Mythical Pokemon's attack. Say, I'd have a Staraptor to shut down Giratina's Shadow Force.

And you missed the most important tips: Save beforehand, and turn off auto save before fighting the Legendary / Mythical Pokemon! They are all one chance only -- at least until Roria League, so these 2 actions guarantee a second chance when rejoined immediately afterwards without saving, in case the Pokemon is knocked out or you run out of Poke Balls.